JP2000144271A - Method of recovering heavy metal from molten fly ash - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for making molten fly ash generated in melting treatment of incineration ash harmless and recovering heavy metals, which is avoided in the use of the large quantity of chemicals and suppressed in the quantity of final waste. SOLUTION: This method comprises (A) a washing process for dissolving soluble components in water by dispersing the molten fly ash in water, (B) a precipitation process for precipitating calcium as a carbonate and dissolving the alkali metals and heavy metals as soluble compounds in the liquid by blowing a high concentration carbon dioxide-containing gas to decrease the pH of the liquid to 5.5-6.5, (C) a process for filtering and separating calcium carbonate generated in the precipitation process with other insoluble components, (D) a process for precipitating the heavy metals as the hydroxide while the compounds of the alkali metals is dissolved in the liquid by adding a water soluble alkaline material in the filtrate in the filtering process to increase pH 9-10 and (E) a process for filtering, separating and recovering the precipitate generated in the precipitation process.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detoxifying molten fly ash generated in a melting process of incinerated ash of municipal garbage and industrial waste and recovering valuable heavy metals.

[0002]

2. Description of the Related Art Fly ash generated during the operation of a municipal garbage incineration facility cannot be discarded unless it has been detoxified by any of concrete solidification, chelation, acid extraction or melting. . Among these methods, the melting treatment requires a melting furnace and requires a large facility cost, but is an excellent treatment method in that the effect of reducing the volume of waste is high. However, since secondary fly ash is generated in the melting step, one of the following must be performed for the treatment.

[0003] The components of the molten fly ash vary depending on the type of refuse to be incinerated, the conditions of the incineration treatment, and particularly when the ash is melted, its melting method. Melting methods include oxygen burner method, coke bed method, plasma method,
There is an electric resistance type. The following table shows examples of the components of the molten fly ash.

Fly ash Na K Ca Zn Pb Si Al Cl A 8.8 1.1 12.7 2.7 0.6 0.5 0.7 47.1 B 4.2 3.1 5.1 11.0 0.7 11.0 1.1 8.5 C 2.4 4.3 41.8 1.4 0.3 0.5 0.3 4.8 D 11.7 20.4 0.5 18.9 4.1 0.7 0.2 36.2 As shown in the table, in general, silica, alumina, silicate, alkali metal and alkaline earth metal chlorides, oxides, carbonates, etc. are the main components of molten fly ash, and zinc, cadmium, tin, Compounds of heavy metals such as lead and copper are added. These heavy metals are high-value substances, and efforts are being made to recover and reuse them from the viewpoint of effective use of resources.

As one of the attempts, MRG (Metal R
ecycling from Garbage) method was proposed. This is a process to recover heavy metals by wet processing, but sulfuric acid,
High consumption of chemicals such as caustic soda and sodium hydrosulfide. Therefore, there is a disadvantage that the amount of generated secondary waste is increased.

[0006] As a means of utilizing garbage incineration ash in other industries, attempts have been made to wash the ash with water to reduce the chlorine concentration, burn it, and add it to a raw material for cement production. In this technique, the waste gas of the cement kiln is used for the purpose of adjusting the pH of the liquid in the washing step, but in order to discharge or recycle the treated water, it is necessary to perform a detoxification treatment.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to avoid the use of large amounts of expensive chemicals in the treatment of fly ash generated by the incineration of municipal garbage and industrial waste, and to achieve the ultimate purpose. An object of the present invention is to provide a method capable of recovering valuable heavy metals while minimizing the amount of waste.

[0008]

SUMMARY OF THE INVENTION There are two embodiments of the method for recovering valuable heavy metals from molten fly ash generated in the melting treatment of incinerated ash of municipal waste or industrial waste according to the present invention. The first consists of the following steps, as shown in the flow chart of FIG. 1: A) a washing step of dispersing molten fly ash in water and dissolving soluble components in water, B) the resulting washing solution Then, a gas containing a high concentration of carbon dioxide is blown into the solution to lower the pH of the solution to a range of 5.5 to 6.5, thereby precipitating calcium as a carbonate, and the alkali metal and heavy metal are soluble compounds. A calcium precipitation step of dissolving in the liquid as
D) an insoluble component separation step of solid-liquid separation together with other insoluble components; D) a water-soluble alkali substance, typically sodium carbonate, is added to the solution obtained in the insoluble component separation process to adjust the pH of the solution to 9 to 10;
And a heavy metal precipitation step in which the heavy metal is precipitated as a hydroxide while the alkali metal compound remains dissolved in the liquid; and E) a heavy metal recovery step in which the precipitate formed in the heavy metal precipitation step is collected by solid-liquid separation. .

A second embodiment of the method for recovering valuable heavy metals from molten fly ash generated in the melting treatment of municipal waste incineration ash according to the present invention comprises the following steps as shown in the flowchart of FIG. 1) a water-washing step of dispersing molten fly ash in water to dissolve soluble components in water; 2) a first solid-liquid separation step of separating water-insoluble components generated in the water-washing step from liquid; By blowing a gas containing a high concentration of CO ₂ into the filtrate in the liquid separation step to lower the pH of the liquid to a range of 8.5 to 10.5, the alkali metal compound remains dissolved in the liquid. A first precipitation step of precipitating calcium as a carbonate and a heavy metal as a hydroxide, 4) a second solid-liquid separation step of separating a precipitate generated in the first precipitation step from a liquid, 5) a first precipitation For sediment in the process Water is added for re-dispersion, and a water-soluble alkali substance, typically sodium carbonate, is added to the dispersion to raise the pH to 11 or more to dissolve heavy metal hydroxides. 6) Dissolution step A third solid-liquid separation step of separating undissolved calcium carbonate from the liquid; 7) a gas containing a high concentration of carbon dioxide is blown into the liquid separated from the precipitate in the third solid-liquid separation step, By lowering the pH again to the range 8.5 to 10.5,
A second precipitation step of re-precipitating heavy metals as hydroxides,
And 8) a heavy metal recovery step of separating and recovering the heavy metal hydroxide generated in the second precipitation step from the liquid.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIGS. 1 and 2, zinc, lead and copper are mentioned as typical examples of heavy metals.
In addition, cadmium, tin, and the like can be similarly recovered.

As the gas containing carbon dioxide at a high concentration, exhaust gases from various burners can be used. If a kiln for producing quicklime by burning limestone or dolomite is nearby, the use of the exhaust gas is particularly advantageous because the concentration of carbon dioxide is particularly high, so that the pH adjustment process proceeds promptly. Exhaust gas from lime kilns is usually 25% by volume
It contains CO ₂ in the degree. This exhaust gas can be used at a temperature from normal temperature to about 400 ° C. In general, the solubility of heavy metal compounds in water increases as the temperature increases. However, the solubility of CO ₂ increases as the temperature decreases, and therefore, an appropriate temperature should be selected in the gas blowing step in consideration of both. In our experience, 20 to 6
Since the range of 0 ° C. is appropriate, the temperature of the exhaust gas is adjusted accordingly.

In the first embodiment, when a gas containing a high concentration of carbon dioxide is blown into a washing solution obtained by dispersing fly ash in water in a washing step, calcium is precipitated as a carbonate in a calcium precipitation step. . When the pH of the liquid decreases and reaches the acidic side (pH of about 6), the heavy metal dissolves and dissolves as ions in the liquid together with the alkali metal compound. In the insoluble filtration step, calcium carbonate is separated together with silica and alumina, which are originally insoluble. When a water-soluble alkali substance such as sodium carbonate is added to the filtrate to raise the pH of the solution to 9 to 10, the alkali metal compound remains dissolved in the solution, but the heavy metal precipitates as a hydroxide. . This is because metals such as zinc, lead, and copper are amphoteric, dissolve in acidic and alkaline conditions, and precipitate as hydroxides from neutral liquids. FIG. 3 shows how the dissolution amounts of these three heavy metal ions change with pH.

This principle is similarly applied to the precipitation of heavy metal hydroxide in the first precipitation step of the second embodiment.

The only process difference between the first and second embodiments is that calcium is separated from heavy metals at an early stage or later. Based on this difference, silica as the insoluble component in the first embodiment,
In the second embodiment, silica, alumina and calcium carbonate can be separately obtained, while a mixture of alumina and calcium carbonate is obtained. As a result, the method and possibility of recycling the insoluble components also differ, and the treatment process should be selected according to the desired recycling method.

In the above two embodiments, the filtrate from the heavy metal filtration step in the former embodiment, the filtrate from the second filtration step and the filtrate from the fourth filtration step in the latter embodiment are all subjected to detoxification treatment. It must be available or released. The filtrate from the heavy metal filtration step and the filtrate from the second filtration step are weakly alkaline liquids containing alkali metal ions and chloride ions that are paired with them, so that they can be rendered harmless by merely diluting as necessary. Since the filtrate in the fourth filtration step is also a solution containing a water-soluble alkali substance such as added sodium carbonate as a main dissolved component, it is neutralized if necessary, but usually becomes harmless only by dilution. The treatment of the filtrate from the second filtration step and the filtrate from the fourth filtration step may be performed for each of the filtrates individually or may be performed integrally.

There is no particular difficulty in the filtration step in the method of the present invention. As the apparatus, a commonly used apparatus such as a thickener, a belt filter, and a filter press may be used.

As described above, the final insoluble matter is a mixture of silica, alumina and calcium carbonate in the first embodiment, and is obtained separately from silica, alumina and calcium carbonate in the second embodiment. You can take appropriate reuse paths. Since these insolubles do not contain harmful substances, they can be used as they are for civil engineering construction materials, or they can be fired or melted by adding another material if necessary, to develop strength and produce aggregates. It is possible to use.

[0018]

Example 1 For molten fly ash having the following component composition,
The recovery of the heavy metals according to the first aspect of the invention was carried out: Na K Ca Zn Pb Cu Si Cl 8.6 1.3 13.0 10.2 0.9 0.1 0.6 43.1% by weight.

10 g of molten fly ash was dispersed in 10 liters of water at a temperature (water washing step), and an exhaust gas branched from the exhaust gas duct of the lime kiln (adjusted to a temperature of 40 ° C.) was blown into the washing solution (calcium precipitation step). ). When the pH dropped to 6, the blowing was stopped and the mixture was allowed to stand for 24 hours, after which the supernatant and the precipitate were separated (insoluble matter separation step). Sodium carbonate was added to the supernatant in which the heavy metal hydroxide was dissolved to adjust the pH to 9.5, and the mixture was allowed to stand for 24 hours (heavy metal precipitation step). These operations were performed in a constant temperature room at 40 ° C. Precipitate obtained by filtration (heavy metal separation step)
Was dried at 120 ° C. and analyzed. The heavy metal content was as follows. (% By weight as metal component)

[0020]

Example 2 For molten fly ash having the following component composition,
According to the second aspect of the present invention was carried out the recovery of heavy metals: Na K Ca Zn Pb Cu Si Cl 2.8 4.7 42.0 1.1 0.5 0.1 2.5 5.1 wt%.

10 g of the molten fly ash is dispersed in 50 liters of water (washing step).
When 0 g was added, the pH rose to 12.5. The washed solution was stirred for 3 hours and then allowed to stand for 24 hours to separate a supernatant and a precipitate (first solid-liquid separation step). The supernatant was branched from an exhaust gas duct of a lime burning furnace, and exhaust gas whose temperature was adjusted to 40 ° C. was blown into the supernatant (first precipitation step). When the pH drops to 9.5, stop blowing.
After standing for 24 hours, the supernatant and the precipitate were separated (second solid-liquid separation step).

50 L of water was added to the precipitate, and sodium carbonate was further added until the pH of the solution reached 12.5 (dissolution step). Then, after stirring for 3 hours and standing for 24 hours, the supernatant was separated (third solid-liquid separation step). Exhaust gas from a lime baking furnace was blown into the supernatant (second precipitation step), and the generated precipitate was separated by filtration (fourth solid-liquid separation step). It dried at 120 degreeC similarly to Example 1, and analyzed. The following results were obtained. (% By weight as metal component)

[0023]

According to the method of the present invention, if the molten fly ash generated as a result of melting the incineration ash of municipal garbage or industrial waste is treated, a large amount of expensive chemicals is used as in the conventional method. Without having to do so, valuable heavy metals can be recovered and reused. As is clear from the graph of the amount of dissolved ions shown in FIG. 3, a high recovery yield can be realized by precipitating the hydroxide of the heavy metal at an appropriate pH. This ensures that the content of heavy metals migrating into the final waste is low, ie the waste is harmless.

By using a gas containing a high concentration of carbon dioxide as a part of the raw material and fixing CO ₂ ,
It is also possible to reduce the amount of O ₂ emission at the same time. This effect is remarkable especially when the exhaust gas from a limestone or dolomite firing furnace is used.

The establishment of a method for treating molten fly ash means that the volume reduction of incinerated ash can be performed more advantageously than before, and the amount of final waste can be reduced. Make use possible.

[Brief description of the drawings]

FIG. 1 is a flowchart showing the steps of the first embodiment of the method of the present invention.

FIG. 2 is a flowchart showing steps in a second embodiment of the method of the present invention.

FIG. 3 is a graph showing the relationship between the amount of heavy metal ions dissolved and pH.

Continued on front page F-term (reference) 4D004 AA37 AB03 BA05 CA13 CA29 CA34 CA35 CA40 CA41 CA50 CC06 4K001 AA06 AA09 AA20 AA24 AA30 BA14 DB07 DB23

Claims (5)

[Claims] 1. A method for recovering valuable heavy metals from molten fly ash generated in the melting treatment of incinerated ash of municipal waste or industrial waste, comprising the following steps: A) A water washing step of dispersing the soluble component in water by dispersing in water, and B) blowing a gas containing carbon dioxide at a high concentration into the obtained water washing solution to adjust the pH of the liquid to 5.5 to 6.5. The calcium carbonate is precipitated as a carbonate, and the alkali metal and heavy metal are dissolved in the solution as a soluble compound. C) The calcium carbonate produced in the calcium precipitation step is
An insoluble component separation step of solid-liquid separation together with other insoluble components; D) a water-soluble alkali substance is added to the solution obtained in the insoluble component separation process to raise the pH of the solution to 9 to 10; Is a heavy metal precipitation step of precipitating heavy metals as hydroxide while remaining dissolved in the liquid; and E) a heavy metal recovery step of solid-liquid separation and recovery of the precipitate generated in the heavy metal precipitation step. 2. A method for recovering valuable heavy metals from molten fly ash generated in the melting treatment of incinerated ash from municipal waste or industrial waste, comprising the following steps: 1) A water washing step of dissolving soluble components in water by dispersing in water; 2) a first solid-liquid separation step of separating water-insoluble components generated in the water washing step from a liquid; 3) CO is added to the filtrate in the first solid-liquid separation step. By blowing a gas containing a high concentration of ₂ to lower the pH of the solution to a range of 8.5 to 10.5, the alkali metal compound is dissolved in the solution, and calcium is precipitated as a carbonate. A first precipitation step in which heavy metals are precipitated as hydroxides, 4) a second solid-liquid separation step in which the precipitate generated in the precipitation step is separated from the liquid, 5) water is added to the precipitate in the precipitation step to redisperse the precipitate. And water-soluble in the dispersion A dissolution step of increasing the pH to 11 or more by adding an alkali substance to dissolve the heavy metal hydroxide, 6) a third solid-liquid separation step of separating calcium carbonate not dissolved in the dissolution step from the liquid, 7) By blowing a gas containing carbon dioxide at a high concentration into the liquid separated from the precipitate in the third solid-liquid separation step, the pH of the liquid is again reduced to the range of 8.5 to 10.5,
A second precipitation step of re-precipitating heavy metals as hydroxides,
And 8) a heavy metal recovery step of separating and recovering the heavy metal hydroxide generated in the second precipitation step from the liquid. 3. The method according to claim 1, further comprising the step of detoxifying the liquid of the second solid-liquid separation step and the filtrate of the fourth solid-liquid separation step separately or integrally so that the liquid can be reused or discharged. Heavy metal recovery method. 4. The method according to claim 1, wherein the heavy metal is at least one of zinc, cadmium, tin, lead and copper. 5. The method for recovering heavy metals according to claim 1, wherein a gas discharged from a limestone or dolomite firing furnace is used as the gas containing carbon dioxide at a high concentration.